This invention relates generally to lasers, and more particularly to optically pumped solid state lasers.
A large number of different kinds of solid state lasers have been discovered, distinguished from one another by host material, by active lasing ions with which the host is doped, and by output characteristics. (See U.S. Pat. No. 4,872,177 issued Oct. 3, 1989 and U.S. 4,860,304 to Mooradian issued Aug. 22, 1989).
Solid state optically pumped lasers comprise, in general, a solid state lasing material sometimes called the gain medium which is disposed within an optical cavity formed between two mirrors. An optical source, such as a laser diode or array of diodes, generates a pump beam which is focused onto the lasing material. Photon energy from the pump laser is absorbed by the gain medium. When a threshold level of absorbed light is achieved stimulated emission of light from the gain medium occurs. In the microchip laser of Mooradian the cavity length, or separation between mirrors, may be as small as 10 to 100 microns to obtain the desirable property that the resulting light oscillates in a single axial mode within the laser cavity.
However, in such microchip lasers and in many conventional lasers it is possible to obtain oscillation in two different polarization modes.
Polarization relates to the direction of the electric field vector E of a light wave. This vector is perpendicular both to the propagation vector k which describes the direction of travel of the wave and to the instantaneous direction of the magnetic field of the wave, H. The direction of the electric field vector is referred to as the "direction of polarization". Randomly polarized light, i.e. light from incandescent lamps, is unoriented and hence unpolarized, whereas light from lasers is generally highly oriented and hence "polarized".
In many lasers, including microchip lasers, two orthogonally polarized modes are present and have nearly the same threshold. This is due to the fact that a 90 degree rotation of a light wave about its propagation vector results in an optical field orthogonal to the original wave which sees the same amount of net gain (gain minus loss). As a result, the laser may oscillate in both polarizations, or may switch polarizations in response to a small amount of feedback provided by optics external to the laser cavity. One way of controlling the polarization of a laser is to introduce a polarizing element within the cavity, such as a Brewster's angle window. While this is a viable alternative for large lasers, it is not an acceptable solution for microchip lasers, which rely on a short cavity length to perform properly. Conventional polarizing elements made to be compact, flat, and thin for microchip applications are often too lossy to use intracavity.
The present invention provides a way of selecting the polarization of a laser without the use of intracavity polarizing elements. In addition it provides a method and apparatus for controllably switching a laser between two orthogonal polarizations. Polarization control of lasers is important because many optical systems (including systems with birefringent elements, gratings, optical surfaces which are not normal to the propagation vector of the light, nonlinear optical elements, or polarizing elements) have different properties for different polarizations of light. By switching a laser between orthoganal polarizations information can be encoded on the output light beam.